Mg(NO.sub.3).sub.2.6H.sub.2 O undergoes a phase transition at about 89.degree. C. (about 192.degree. F.) releasing or alternately absorbing a substantial quantity of latent heat of fusion. As such, the material has advantageous properties for use as an encapsulated heat storage medium for use in conjunction with solar energy systems for hydronic heating and off peak electrical power heat storage applications. In order to improve the material for such applications, however, there is a need to develop additives or a series of additives to modify the supercooling properties of hydrated Mg(NO.sub.3).sub.2 compositions, and most advantageously, additives which are highly effective in a very small amount.